The present invention relates to blade clearance arrangements and more particularly to blade clearance arrangements used with regard to shrouded or shroudless turbines within a gas turbine engine.
Referring to FIG. 1, a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, a combustor 15, a turbine arrangement comprising a high pressure turbine 16, an intermediate pressure turbine 17 and a low pressure turbine 18, and an exhaust nozzle 19.
The gas turbine engine 10 operates in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produce two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high pressure compressor 14 is directed into the combustor 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbines 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by suitable interconnecting shafts.
In view of the above, it will be appreciated that the control of the gap between the tips of blades and an outer casing is important in order to achieve efficiency with respect to the turbine operation as well as to avoid problems such as tip oxidation reducing blade life particularly when utilised with the shroudless turbines. Furthermore, with higher loading upon turbines it will be appreciated that accurate control of tip clearance becomes more important. Nevertheless, there is a continuing requirement to achieve leaner combustors and a reduced blade count so that the use of shroudless turbines is at least more convenient. It will be understood that inherently due to variable rotational speeds, loading and temperature cycling, blade tip clearance gaps will vary through operational cycling of a gas turbine engine. Techniques have been developed for monitoring the gap width between blade tips and an outer casing but convenient means for adjusting the gap are less readily available. Thus, the feedback control mechanism which monitors the gap width can be specified in order to achieve accurate tip clearance control and so enable high efficiency and performance retention in an engine but there are limitations with respect to the accuracy with which the clearance gap can be adjusted or maintained.
Previous approaches to tip clearance control have either depended upon scheduled thermal processing or pneumatic systems. For example, a scheduled thermal tip clearance gap control uses a passive inner ring and controlled thermal expansion of a carrier ring to move nozzle guide vanes and attach seal elements radially in and out relative to a blade. Another example is of a thermally driven single skin casing which is locally cooled with bypass air to change the radial position of the seal elements attached to it and so adjust clearance gaps between that casing and blade tips. In a pneumatic arrangement, air pressure behind seal segments is rapidly reduced to drive the segment out prior to a desired operational requirement for the engine, such as a particular aircraft manoeuvre, and then the segments are recovered to their original position after a time. In such circumstances these approaches are scheduled, that is to say they respond to a manual control signal or adapt thermally due to a desired operational requirement such as throttle setting. Normally tip clearances in the order of 0.020″ to 0.035″ are achieved, but this can vary over the course of engine life and flight cycling.
The above thermal and pneumatic arrangements are typically too crude for acceptability with regard to shroudless turbine blade arrangements. Potentially clearance gap widths of less than 0.01″ would be desirable in order to achieve turbine efficiency and an acceptable blade life in a gas turbine engine. Furthermore, in order to achieve higher efficiencies, it would be desirable to control tip gap clearance throughout all engine operational cycles and, it will be understood that for feedback control, previous arrangements have difficulties with regard to compensation for expected or predictable changes in blade or segment condition, particularly under transient conditions.